1. Field of the Invention
This invention relates to a plugging apparatus for fixing a mechanical plug in a defective heat transfer tube of a steam generator or heat exchanger to bring the tube out of use.
2. Description of the Prior Art
A steam generator or heat exchanger installed in nuclear power plants and the like includes a large number of thin heat transfer tubes, and an end of each of the heat transfer tubes is inserted in a corresponding through-hole of a tube plate and is secured thereto. A heating medium (primary cooling water) heated by a heat source such as a reactor is passed through each heat transfer tube, so that heat exchange takes place between the heating medium and secondary cooling water flowing around the heat transfer tubes.
Such heat transfer tubes may be damaged by corrosion and the like, thereby resulting in water leakage. For preventing water leakage from a damaged heat transfer tube, a so-called mechanical plug repairing method is carried out in which, as shown in FIGS. 3 and 4, a mechanical plug (hereinafter referred to as a plug) is inserted into the heat transfer tube surrounded by a tube plate 3 and is sealed therein thereby preventing the primary water from flowing into the transfer tube 1. This method is carried out to bring the damaged heat transfer tube out of use. The details of this method are disclosed in U.S. Pat. No. 4,390,042 (corresponding to Japanese Patent Laid-Open No. 57-52799).
Specifically, as shown in FIG. 3, a plug body 7 of the plug 5 is cylindrical with one end closed, and a plurality of circumferential grooves 9 are formed on its outer peripheral surface at adequate intervals in the axial direction. The inner peripheral surface of the plug body 7 is tapered such that the inner diameter decreases progressively as approaching the open end. A cone 13 having a threaded through-hole 11 formed at its center is previously placed in the plug body 7.
A handling rod 15 is screwed into the threaded hole 11 of the cone 13 and is then pulled with a strong force in the direction of the arrow by a plugging apparatus (not shown). Consequently, as shown in FIG. 4, the plug body 7 is radially expanded and pressed against the inner surface of the heat transfer tube 1, so that the end of the heat transfer tube 1 is mechanically sealed with the plug 5. Then, the handling rod 15 is detached from the cone 13.
FIG. 5 is a simplified sectional view of a conventional plugging apparatus. As will be appreciated from this drawing, this plugging apparatus 17 includes a plugging cylinder 21 with a hollow shaft piston 19, and the handling rod 15 extends through the hollow shaft piston 19. The handling rod 15 is threadedly meshed with a threaded inner portion 23 of the hollow shaft piston 19, and by rotating a wheel 25 attached to the lower end of the handling rod 15, the handling rod 15 can be moved vertically in relation to the hollow shaft piston 19. The handling rod 15 has a threaded portion 27 at the upper end thereof which will be screwed into the threaded hole 11 in the cone 13 of the plug 5.
When putting the plug 5 in the heat transfer tube 1 using the foregoing plugging apparatus 17, first, the threaded upper portion 27 of the handling rod 15 is screwed into the threaded hole 11 in the cone 13, and while holding the plugging cylinder 21 in contact with the lower side of the tube plate 3, the plug 5 is inserted into the heat transfer tube 1. After the position of the plug 5 is settled, a stopper 29 threadedly fitted to the handling rod 15 is turned until it comes into contact with the lower end of the hollow shaft piston 19. Then, pressurized oil is supplied through a port 31 in the plugging cylinder 21; as a result, the hollow shaft piston 19 is moved down accompanied with a downward movement of the handling rod 15 and the cone 13. Consequently, the plug body 7 is radially expanded as described above, whereby the heat transfer tube 1 is sealingly plugged. Finally, the wheel 25 is turned to detach the handling rod 15 from the cone 13, whereby the plugging work is completed.
Similar plugging apparatus are disclosed in U.S. Pat. No. 4,369,662 (corresponding to Japanese Patent Laid-Open No. 57-151494), and U.S. Pat. No. 4,787,420 (corresponding to Japanese Patent Laid-Open No. 63-143495), etc.
Although making use of pressurized oil to pull the cone, the conventional plugging apparatus must be handled directly by a worker in a water chamber of the heat exchanger.
Therefore, although the plug will be reliably fixed, such repairing work is subject to severe restrictions when applied to the heat exchanger of a nuclear power plant. Namely, the working time of each worker in the water chamber is limited, since the inside of the water chamber is filled with the radioactive primary coolant and may be contaminated with radioactivity.
In other words, a large number of workers must be engaged since the level of exposure of each worker must be limited for health reasons.